Cargo tanks which handle volatile liquids must be provided with relief valves to permit the interior of the tank to be vented to the atmosphere if the pressure inside of the tank increases to an excessive level due to heat exposure, for example. However, in the event the tank is rolled over during a vehicular accident, it is desirable that the relief valve not be allowed to open during the instant that a high pressure, sudden liquid surge arises within the tank as it strikes the ground. If the valve were allowed to open at that time, the surging liquid could spray and spew the volatile contents over a wide area in the vicinity of the rollover and present a very dangerous situation. On the other hand, once the sudden liquid surge has passed, it is important that the relief valve then be capable of performing its venting function so that the pressure within the tank can be gradually released if it exceeds the relief pressure for any prolonged period of time.